Recently, interests in aerogel have been increased as the industrial technologies become more cutting-edge. The aerogel has a porosity of 90% (percent) or more and a specific surface area of several hundreds up to about 1500 m2/g (square meter/gram), but the specific surface area may be varied, depending on the starting materials. Also, the aerogel is an advanced material having a nanoporous structure. Therefore, the nanoporous aerogel may be used in the application field such as a very low dielectric, a catalyst, an electrode material, a soundproofing material, etc. In particular, the silica aerogel is a very effective ultra heat insulating material that may be used in the fields of a refrigerator, an automobile, an airplane and the like since the silica aerogel has a low heat conductivity characteristic. This aerogel may be prepared according to various known methods. For example, WO95/06617 discloses that a hydrophobic silica aerogel is prepared by allowing a water glass to react with sulfuric acid at pH 7.5-11, washing the formed silica hydrogel with water or a diluted aqueous solution of inorganic salts (sodium hydroxide or ammonia) at pH 7.5-11 so as to remove ion components from the silica hydrogel, removing water with C1-5 alcohol and drying the silica hydrogel at 240 to 280° C. (centigrade) under a supercritical condition of 55 to 90 bar. In the method, the supercritical drying process is carried out without undergoing a silylizing process.
WO96/22942 discloses a method for preparing an aerogel, comprising: providing a silicate lyogel; subjecting a solvent exchange process with other organic solvents (methanol, ethanol, propanol, acetone, tetrahydrofuran, etc.), if necessary; allowing the silicate lyogel to react with at least one of silylizing agent that does not include chlorine component; and drying the silicate lyogel under a supercritical condition. In the method, the solvent exchange is carried out prior to the silylizing process, and subject to the supercritical drying process.
WO98/23367 also discloses a method for preparing an aerogel, comprising: allowing acid to react with a water glass to form a lyogel; washing the lyogel with an organic solvent (alcohol (methanol, ethanol), acetone, ketone, etc.) so that a content of water can be less than 5% (percent) by weight; and followed by undergoing silylizing and drying processes. However, the solvent exchange process is carried out prior to the silylizing process.
WO97/17288 also discloses a method for preparing an aerogel, comprising: preparing a silicic acid sol having a pH of 4.0 or less from an aqueous sodium silicate solution using at least one organic and/or inorganic acid; separating salts which are formed from the acid and sodium silicate cations from the silicic acid sol at 0 to 30° C. (centigrade); polycondensing the resultant silicic acid sol to form an SiO2 gel by the addition of a base; washing the obtained SiO2 gel with an organic solvent (aliphatic alcohol, ether, ester, ketone, aliphatic or aromatic hydrocarbon) until the water content of the gel is ≦5 wt. % (weight percent); surface-silylating the obtained gel and drying. However, the solvent exchange process is carried out prior to the silylizing process.
WO97/13721 discloses a method of replacing water with C1-6 aliphatic alcohol in hydrogel particles; removing an organic solvent from a hydrogel using C1-3 alcohol, diethylether, acetone, n-pentane and n-hexane and the like; and drying wet gel at a temperature between a boiling point and a decomposition temperature of a solvent at an atmospheric pressure, and at a pressure less than a supercritical pressure of a solvent. This method is a technique that is associated with the drying at an atmospheric pressure but does not include a silylizing process. However, the method has problems that the process is complicated since it requires a 2-step solvent exchange process of replacing water with a polar solvent (butanol, etc.) and replacing the polar solvent with a non-polar solvent (pentane, etc.) for the purpose of the drying at an atmospheric pressure.
WO98/23366 discloses a method for preparing an aerogel, comprising: forming hydrogel at pH 3 or more; undergoing intermediate processes; mixing a hydrophobic agent and the hydrogel to modify a surface of the hydrogel; washing the modified hydrogel with a protic or aprotic solvent (aliphatic alcohol, ether, ester, ketone, aliphatic or aromatic hydrocarbon, etc.) or a silylizing agent, when necessary; and drying the washed hydrogel, and also discloses a for preparing an aerogel capable of being performed without undergoing the solvent exchange process since the exchange of water with other solvents is a time and energy-consuming process.
Also, there is a technique of removing moisture from silica by using butanol (n-butanol), propanol and their mixture to prepare silica whose particles have a nanosize (Korean Patent Application No. 2004-72145), and the technique comprises: precipitating a silica by adding HCl to a water glass so as to facilitate the reaction; mixing the water glass with butanol and the like; filtering and distilling the resulting mixture to remove moisture from the silica; and drying the mixture at a high temperature of 285° C. (centigrade) to prepare silica particles having a nanosize. The silica surface may be endowed with hydrophobicity since a hydroxyl group of a silica surface is changed with a butoxy group through the reaction with butanol in a solvent exchange process and a subsequent drying process. However, since the inverse reaction in the silica surface is possible through the reaction with moisture in the air, the butoxy group is exchanged with a hydrophilic group, which makes it difficult to endow the silica with permanent hydrophobicity. Therefore, there are limits to use the silica, which has been prepared according to the above-mentioned methods, in the field of the permanently hydrophobic silica particles.
Korean Patent Application No. 2006-87884 proposes a method comprising: adding a water glass (sodium silicate) to HCl to form a silica gel under an acidic condition of pH 3-5; washing the formed silica gel with distilled water and filtering the washed silica gel; mixing 1-30% (percent) by weight of a silylizing agent, such as hexamethyldisilane, ethyltriethoxysilane, triethylethoxysilane, ethyltrimethoxysilane and methoxytrimethylsilane, with 70-99% (percent) by weight of alcohols (methanol, ethanol, propanol, etc); refluxing the resulting silica gel 4-12 hours to modify a surface of the silica; filtering the surface-modified silica; and performing a solvent exchange of the silylized silica gel with n-butanol to remove moisture and reaction residues from the silica gel at the same time. However, the method has disadvantages that it is impossible to recover the used silylizing agent and alcohol solution since the silylizing agent remains mixed with the alcohol solution, and the cost of the products is very high since the expensive silylizing agent is used in a large amount, and thus it is unfavorable in respect of economical efficiency.
Korean Patent Application No. 2006-98643 briefly describes the context of the previous patent application as one step, and proposes a method comprising: adding a water glass (sodium silicate) to HCl to form asilica gel under an acidic condition of pH 3-5; washing the formed silica gel with distilled water and filtering the washed silica gel; adding a silylizing agent, such as hexamethyldisilane, ethyltriethoxysilane, triethylethoxysilane, ethyltrimethoxysilane and methoxytrimethylsilane, to n-butanol solution; and silylizing the silica gel and removing moisture and reaction residues from the silica gel at the same time. However, the method needs a reaction time from at least 4 hours up to 24 hours, and requires multiple processes of filtering the silica gel after the reaction process and recovering the solvents and the silylizing agent from the separated cake in a separate drying system so as to obtain the final silica particles. Also, the method has a problem that about 10% (percent) of butanol may be inevitably lost according to a solubility curve of water and butanol as shown in FIG. 3. Butanol is expensive, but the silylizing agent that is expensive several hundred times more than the butanol may be lost with its being dissolved in the butanol, and therefore the method also has very serious problems in respect of economical efficiency. Recycled solvents may be degraded in quality since a relatively large amount of water is dissolved into the recovered butanol solvent. Furthermore, there is a limit to the aerogel having a relative low specific surface area, that is, a specific surface area of 600-700 m2/g (square meter/gram), which is referred to as one disadvantage of the method in which a water glass is used instead of TEOS as a starting material.
Accordingly, the present invention is designed to solve such drawbacks of the prior art, and therefore an object of the present invention is to provide a method for preparing a permanently hydrophobic silica aerogel powder having a higher specific surface area, the silica aerogel powder being prepared by preparing a wet gel having a higher specific surface area at an atmospheric pressure condition using a surfactant, and subjecting the wet gel to a one-step process at a low temperature in a short time, wherein the method is able to recover up to 96% (percent) of the used solvents and silylizing agent.
Also, another object of the present invention is to provide a permanently hydrophobic aerogel having a high specific surface area, prepared according to the method of the present invention.